There are many applications which could benefit from a reduced size actuator, but which can induce a significant stroke and apply a significant force.
Motors are most commonly used as actuators for generating large strokes and forces. For many miniaturized applications, even small motors are too large, heavy, noisy and expensive.
Shaped memory materials (SMMs), especially shape memory alloys (SMAs), are able to provide significant force and stroke when heated beyond their specific phase change temperature. Even if the dimensions of the material are small, the force and stroke delivered are, relative to these dimensions, very high and accurate, over a very long period of time and after many switching operations.
The use of SMM to provide an actuation signal, dependent on temperature, has thus been investigated. For example, U.S. Pat. No. 6,764,166 discloses the use of a SMA forming part of an ejection nozzle for an ink jet printing device. After the temperature rise and shape change due to the phase change, the material must be brought back to the original shape, before the actuation can be restarted. In U.S. Pat. No. 6,764,166, the nozzles are pre-stressed into a curled state, and heating of the nozzle cause the nozzle plate to straighten. After cooling, the pre-stress returns the plate to the curled state.
The need for pre-stressing is a disadvantage which follows from the fact that when there is a temperature decrease, the phase changes back to the original phase, but the shape does not. Thus, before the actuator can be used again, after a temperature decrease, an external actuation must be initiated to reverse the shape change of the SMM.